This invention relates to cooling dies for use in association with food extruders in the manufacture of texturised protein food products, and a perforated die plate for use at the cooling die outlet.
In particular, the present invention relates to a cooling die for use in the manufacture of an extrude food product that has the appearance of fibrous meat pieces such as fish, chicken, lamb or beef. The cooling die is attachable to the outlet of an extruder which may contain one or more screws and feeds molten extrudate to said cooling die at a temperature between 110xc2x0 and 180xc2x0 C.
Various protein texturisation processes have been used for some time in the manufacture of various food product, such as in the manufacture of sausages, cheese curds, mozzarella processed cheeses, bakery products, tofu, kamaboko, meat analogs and seafood analogs. A fibrous texture may be obtained by various means, including extrusion cooking at low moisture levels (typically 10-30% by weight).
Extrusion cooking at high moisture levels (e.g. typically 30 to 80% water by weight) is a relatively new technique which is finding use mainly in the field of texturisation of protein food products.
High moisture extrusion cooking has been discussed as a means of restructuring various natural protein sources, such as fish mince, surimi, de-boned meats, soy flours, concentrates, cereal flours, dairy proteins and the like, in order to obtain cohesive fibrous or lamellar structures (e.g. see xe2x80x9cNew Protein Texturisation Processes by Extrusion Cooking at High Moisture levelsxe2x80x9d by J C Cheftel et al, Food Reviews international, 8 (2), 235-275 (1992) published by Marcel Dekker, Inc.).
Unlike low moisture extrusion cooling, high moisture extrusion cooking requires the use of cooling dies for cooling, gelling and/or solidifying the food product issuing from the food extruder. A cooling die dissipates the thermal and mechanical energy accumulated in the food mix, increases the viscosity of the mix, and prevents product steam flash at the die outlet
The concept of extruding cereal, meat or other protein blends at high moisture through an extruder and then passing the extrudate through an attached cooling die, so that product exits the cooling die at temperatures not exceeding 100xc2x0 C. (typically about 80xc2x0 C.), is not a new one. This cooling of the product is quite important in order to eliminate expansion of said product as a consequence of steam flashing, amongst other things. There are numerous patents and articles discussing this subject, including discussions of die design in particular.
It is understood that texturisation of the protein food product takes place during cooling as a result of lamellar flow in the die.
Three main types of cooling dies are known for use in this field of technology/application. Most commonly known are elongated rectangular cooling dies. A rectangular cooling die has a long rectangular prismatic housing in which is received a rectangular duct extending along the length of the die. The regions surrounding the rectangular cavity (duct) are cooled with water thereby enabling the extruded food product passing through to be cooled. Cooling dies may also be cylindrical with an internal cylindrical cavity extending along the length thereof. Such a cooling die functions in much the same manner as a rectangular cooling die. There are annular cooling dies in which the internal cavity has an annular cross-section defined by an inner core and an outer cylinder. The inner core and outer cylinder are cooled, thereby enabling the food product passing through the cavity to be cooled.
One problem with known cooling dies is that, as portions of the food product come in contact with the coded surfaces of the die, these portions become thicker, tend to stick to the surface of the die and slip at a lower rate than internal sections of the product Accordingly, velocity gradients and shear forces develop which may cause inconsistencies in the food product and problems with the smooth continuous operation of the cooling die and extruding apparatus. This is a particular problem where the dimensions of the cooling die cavity (e.g. height, width and/or length) have been increased so as to achieve greater throughput of extruded products.
Another problem with known cooling dies is that they effectively cause a xe2x80x9cbottle-neckxe2x80x9d in the extrusion process. Typically, the capacity of a commercial cooling die is about 100 kilograms per hour, so that product output is limited to this value.
Whilst this extrusion rate has been found to be desirable in order to achieve a commercially acceptable product, it is desired to have greater product output rates to increase yields. The production of high moisture extruded products at manufacturing outputs of up to 200 kg/hr using single channel cooling dies has also been documented. However, extruded products manufactured at these rates tend to be of lower quality and/or consistency than those manufactured at lower rates. Production rates in excess of 200 kg/hr are much more difficult to achieve, due to the physical limitations of known cooling die designs.
The output of a cooling die is determined by a multitude of factors, one major factor being the capacity (of volume) of the cooling die cavity (or channel) which is determined by the cross-sectional area and the length of the die cavity, it increased production rates are required, one has the choice of increasing the cross-sectional area or die length or both. This strategy however may be limiting. For instance, the cross-sectional area of the die cavity is largely determined by the desired product characteristics. Also, increasing the cross sectional area would typically increase the amount of time required to cool the product. It may result in inconsistencies in the product due to the outer portions of the extruded product cooling much faster than the inner portions. Altering the die shape may give a product not meeting desired visual parameters. Increasing the length of the cooling die also has limitations due to the fact that the pressure drop along the die is proportional to the length of the die. Increasing the pressure drop along said cooling die will decrease output of the die or require increased extruder capabilities.
Attempts have also been made to increase the capacity of cooling dies through the use of higher flow rates with cooling dies of greater cross-sectional areas. This measure necessitates longer cooling dies. This has a number of adverse consequences. For instance, longer cooling dies increase the likelihood of inconsistencies arising in the food product and structure blockages occurring in the cooling die. Also, such dies obviously take up more area or floorspace of the production plant, which in turn increases costs.
Japanese patent application No. 4-214049 (publication No. 6-62821) discloses a multi-channel cooling die which is used in the extrusion of thin, thread-like food products from high moisture content proteinaceous raw materials. The cooling die is essentially constructed like a typical shell-and-tube heat exchanger, wherein the shell covers at the axial ends of the cylindrical shell are replaced with purpose built end plates. The inlet end plate is flanged to the extruder""s die plate holder, while the other end plate is similar in layout to the stationary tube sheet of the heat exchanger, i.e. a multiple-orifice plate in which the ends of the plurality of inner tubes are wedged and supported.
The plurality of thin-walled inner tubes employed in such typo of cooling die ensure efficient cooling at higher through-put rates of extrudate. It is said that the individual tubes possess high pressure resistance thereby enabling processing of greater amounts of raw materials as compared with conventional, single cavity cooling dies.
One serious shortcoming of such type of cooling die is the need to use xe2x80x9cpigsxe2x80x9d or long rods for cleaning the individual inner tubes through which the extrudate flaws during processing. The smooth surface of the tubes can be damaged during the cleaning process (due to their length), which may result in irregular loading of individual tubes from the extruder as a consequence of increased surface roughness (and back pressure) at individual tubes. Also, in case one of the tubes is damaged to an extent that it no longer provides a flow path for the molten extrudate. It is necessary to replace the entire cooling die or perform a time and labour intensive replacement process: because the individual tubes are received in airtight manner at the end plates of the cylindrical shell, all tubes have to be removed and refitted in order to exchange any one of them.
The present invention is directed to providing a cooling die for use with a food extruder, which enables greater manufacturing output without substantially increasing the cross-sectional area or length of the extrudate flow cavity, when compared to single cavity cooling dies used in the art, by providing a multi-channel cooling die which addresses some or all of the disadvantages perceived to exist with shell-and-tube type cooling dies.
The present invention also seeks to provide a cooling die which incorporates means to enable extrusion of extrudates of varying cross-sectional shapes/sizes without the need to stop extrusion of the product.
According to a first aspect of the invention there is provided a cooling die for extruding of high moisture extrudate food products, having a cooling die body in which are defined a plurality of extrudate flow channels extending between and inlet end of the cooling die that is attachable to an extruder and an outlet end for delivery of cooled-dawn extrudate, and coolant cavities located in heat-exchanging communication with the extrudate flow channels and connectable to a source of coolant, characterised in that the cooling die body consist of a plurality of thick plates having a plurality of first and second bores extending between and opening at the planar surfaces of the thick plates, and in that the plurality of thick plates are stacked plane-parallel and fastened together such that the opening of the first and second bores of adjoining plates are respectively aligned with one another, whereby the first bores form said plurality of extrudate flow channels and the second bores form a plurality of discrete coolant channels extending through the length of the cooling die body.
Such type of cooling die layout has a number of advantages over the above described shell-and-tube cooling die. Firstly, the stacked arrangement of individual plate members allows assembly of cooling dies of varying length by removing or inserting individual plate members, thereby providing adaptability to different cooling requirements of the extrudate. Secondly, the extrudate flow channels are easier to clean without risk of damage, as the die body can be easily dismantled thereby to provide access to the relatively short bores formed in the individual plate members. Conventional sealing elements and/or mean are provided between the individual plate members thereby to ensure formation of leak proof and pressure resistant channels extending between the axial ends of the die body assembly once the plates have been stacked and secured to one another.
There are numerous ways in which the stack of plate member can be secured to form a unitary die body, including fastening of adjoining plate members through suitable fasteners (i.e. recessed screw/nut fasteners), clamping of the entire stack of plates between end plates which are tensioned using threaded rods or the like, and similar. Also, alignment elements may advantageously be present between each pair of adjoining plates to ensure coaxiality of the openings of the first and second bores of the plates with one another. There are numerous ways in which the plates can be fastened and aligned with one another, as is the case with sealing mechanisms to provide leak-free passage of extrudate and coolant through the respective channels in the cooling die body. These am known to the competent tool making engineer.
In a preferred form of cooling die in accordance with the present invention, there are provided a coolant supply and a coolant discharge end plate at axially opposite ends of the cooling die body, wherein the end plates include manifold conduits for supplying or discharging coolant to/from the coolant channels of the die body, as the location of the end plates dictate. Advantageously, the manifold conduits communicate with a common coolant supply/discharge armature fixed to the respective end plate for connecting the manifold conduits to a source of coolant or a receptacle reservoir, as the case may be. One manifold conduit may be arranged to supply or receive coolant from one or a plurality of coolant channels, the latter being grouped in fluid communication in sets of two or more channels in order to decrease individual connection points between manifold conduits and coolant channels. Extrudate flow bores will also be present in both end plates, to allow entry and exit of extrudate into thee cooling die body plates
The extrudate flow channels of the die body may be orientated in any suitable manner and extend either axially through the body or in a helical pattern or the like. In case of co-axially arranged extrudate flow channels, these may be arranged in a regular or irregular pattern of rows or columns, in a preferred embodiment the extrudate flow channels being located radially about the longitudinal axis of the cooling die body.
In a preferred form, the cooling die longitudinal axis is disposed in alignment with the axis of the extruder to which the former is connected in use.
In one preferred form, the cooling die body contains twenty-four (24) extrudate flow channels equidistantly spaced about the axis of the die, The arrangement of extrudate flow channels in a pattern that is equidistantly spaced about the axis of the die with each channel having a cross-section opening which extends substantially in radial direction, has several advantages, including efficient use of space, ease of manufacture of the individual plate members and optimal packaging density. This arrangement also allows to interleave one or more coolant flow channels between neighbouring extrudate flow channels.
In a preferred form, the extrudate flow channels will have an oblong or long-hole cross-section (i.e. rectangular shape with rounded short ends) thereby to prevent sharp edges in which extrudate could deposit and adhere. Each extrudate flow channel will have a radial height which is substantially greater than the width thereof (i.e. dimension in peripheral direction of the cylindrical die plates). The height of each channel is preferably greater than 20 mm and typically about 70 mm, whereas the width would be about 4 mm or more, preferably about 8 mm. It will be appreciated that other cross-sectional shapes may be used instead of substantially rectangular cross-sections, bearing in mind that different cooling requirements apply to different cross-sectional shapes of extrudate flow channels.
A noted above, it is preferred to have an alternating arrangement of coolant and extrudate flow channels, wherein it is advantageous to have two or more radially spaced coolant channels extended between two neighbouring extrudate flow channels, thereby to increase heat transfer from the extrudate into the coolant. A radially symmetrical arrangement of coolant and extrudate flow channels about the longitudinal axis of the cooling die body is preferred.
Due to the operating pressures and temperatures, the thick plates that make up the cooling die body will be machined from solid metal, e.g. Stainless steel, aluminium and the like.
In a further development of the present cooling die invention, there is incorporated an extrusion die plate at the outlet end of the cooling die body downstream of the coolant discharge header plate (also referred to as a distribution end plate) the extrusion die plate having a plurality et discharge orifices of predetermined shape and configuration that are grouped and arranged to be selectively bought in axial alignment with predetermined ones of the extrudate flow channels thereby to enable extrusion of cooled-down extrudate bands having selected ones of different cross-sectional shapes in accordance with the discharge orifice shape.
Incorporation of such type of cooling die extrusion plate allows extruding of extrudate bands having selected cross-sectional shapes through a single extrusion plate by simple repositioning of the extrusion die plate at the cooling die outlet end. This in turn enables to increase efficiency of the extruder cooling die assembly, as the need to shut down the extruder (often for a few hours) in order to exchange a die extrusion plate is avoided.
In a preferred form of extrusion die plate, the number of discharge openings is a natural multiple of the number of extrudate flow channels, wherein the respective multiples are grouped together such that one group can be brought into alignment with the extrudate flow channels, whilst the other group is offset therefrom, the first group of discharge orifices having a cross-sectional shape that is different from that of the second group. Alternatively, the discharge orifices may all have the same cross-sectional shape and selected ones of the openings may be traversed by a predetermined number of cutting blades, wires or webs thereby subdividing the respective orifice into a corresponding plurality of smaller openings.
Preferably, the extrusion die plate is arranged to move between a first position in which the orifices featuring the cutting elements align with a selected number of the extrudate flow channels, and a second position, in which the orifices featuring the cutting elements do not align with the channels. A benefit of this embodiment is that one can selectively have extruded product exiting the cooling die either as wide strips (i.e. where the extrusion die plate orifices have an oblong or rectangular cross-section corresponding to that of the extrudate channels) or as xe2x80x9cstringsxe2x80x9d (for instance, having squarish cross-sections), simply by moving the die plate (or screen) from the first position to the second position. Rapid change of the extrusion die plate position at the outlet of the cooling die results in less down-time and less wastage of product, which thereby results in considerable cost savings.
When the multi-channel cooling die is a xe2x80x9cradialxe2x80x9d cooling die (having extrudate flow channels arranged equidistantly about the axis of the die body and each having a substantially radial extension), extrusion the die plate is preferably disc-shaped and rotatable about the central axis of the assembly. The circular plate may be moved between the first position and the second position simply by rotating it. The plate may have sets of orifices featuring cutting elements adapted to align with each of the extrudate flow channels and xe2x80x9copenxe2x80x9d orifices between each of the sets of apertures featuring cutting element. When the set of cutting elements are aligned with the extrudate flow channels, the product exiting the cooling die will be cut by the cutting elements so as to form xe2x80x9cstringsxe2x80x9d of extruded product
Where, for example, the cooling die has twenty four equi-peripherally spaced extrudate flow channels, the extrusion die plate may have a corresponding twenty four orifices with cutting elements and twenty four xe2x80x9copenxe2x80x9d orifices. By rotating the plate or screen through 7.50xc2x0, the plate is moved from the first position, in which the set of apertures featuring cutting elements align with the extrudate flow channels, and the second position, in which the xe2x80x9copenxe2x80x9d apertures align with these channels.
Alternatively, in the case of a twenty four channel cooling die, the cooling die extruder plate or screen may have twelve apertures featuring cutting elements and twelve xe2x80x9copenxe2x80x9d apertures. In this case, an additional xe2x80x9cshutterxe2x80x9d plate would be preferably located at the inlet end of the stacked die body, to selectively shut 12 of the extruder flow channels so that product would only be permitted to pass through the other twelve of the channels; the apertures featuring cutting elements, or the open apertures would, selectively, be in alignment with these open twelve channels. The apertures featuring cutting elements or the xe2x80x9copenxe2x80x9d apertures which are not in use would be in alignment with those channels through which no product is passing,
As will be appreciated, provision of such variable cooling die extruder plate requires that the cooling die end (or coolant distribution) plates at each and of the cooling die body are designed with axially extending sack holes for the coolant channels of the die. The sack holes are then in communication with the radially extending bores that terminate in the peripheral surface at suitable fitting that enable connection to coolant manifold lines. Accordingly, the cooling fluid will not flow axially past the end plates of the cooling die.
The cooling die may further be associated with a cutting apparatus that includes cutting means for cutting the strips or lengths of the extrudate exiting the cooling die extrusion plate into pieces of desired length. This cutting means may be a rotatable blade. Preferably, the cutting apparatus further includes means for varying the speed at which this blade rotates. By varying this speed, the length of pieces of the product being cut can thereby also be varied.
A preferred embodiment of the invention is described below with reference to the accompanying drawings and by way of example only. Further advantages and preferred features of the invention are discussed there also.